First, the only way to measure your efforts is by doing a set distance. Different courses will rend different results, but in the end that does not matter. What matters is, are you improving?
Second, 100k seems to closely resemble the dedication and time required to do a running marathon. It’s not ridiculous, and it’s not easy.
Third, rides longer than 100k may be epic and wonderful and all that… but 100k seems about right when it comes to getting in a killer work out still having some energy left over.
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